Fluvial processes and channel morphometry of the upper Orashi basin in southeastern Nigeria

Abstract

This study is an attempt to document the hydraulic characteristics of the Orashi channel and to analyze its similarity or otherwise to other channels. Results show that the Orashi River has a general increase in width, depth and discharge downstream while velocity decreased downstream and that the exponents for the hydraulic geometry of the Orashi are; 1.08, 0.52, and -0.56 respectively for exponents b, f and m. These results are quite different from theoretically derived estimates. The relationship between the channel processes operating in the Orashi catchment and its morphology differs rather markedly from earlier studies and generalizations. While it attains some measure of channel equilibrium between morphology and hydrology, the Orashi channel is not well adjusted to the ‘normal’ flow of the hydrologic regimen. This may be explained by the presence of the Oguta Lake which acts as a local base level and the flow of the Orashi into the alluvial plains of the River Niger which affects considerably the morphologic and hydrologic character of the Orashi. Also, bed-load analysis of Orashi channel sediments indicated that 79% of material transported by the river is finer than the 300 micron and this highly cohesive nature of the Orashi channel perimeter sediments contributes significantly in influencing the (F) ratio of the Orashi channel.

KeyWords: Orashi, Drainage basins, watershed morphology, morphometric analysis, Nigeria